Half of msrp is a good starting point. They can go above that if they are fairly new and/or are in high demand. Condition plays a roll of course. If old and/or in low demand they could go for less than half of msrp.
Another trend I see is the more expensive an item is originally the more likely it will sell used for less than half of msrp. I'm guessing the reason is that there are less people that can afford such an item therefore less demand.
Just place an ad and start out a little high on price. If they are in demand then they should sell quickly. If not then lower the price a little and go from there.
If you are not in a hurry to sell them then you can hold out for more money.
